This invention relates to furnaces, and more particularly to wood-burning, downdraft furnace.
A simple wood-burning furnace usually comprises a metal box having a door for loading wood, an air inlet control system (often part of the door) for controlling the amount of combustion air admitted into the box and an exhaust flue for directing exhaust gases from the box. Such simple wood burning stoves tend to be inefficient because unburned vapors and particulates pass out of the exhaust flue. Admitting more combustion air may reduce the amount of unburned vapors and particulates passed out of the exhaust flue, but then the fire tends to burn too hot and too fast.
A solution to such problems is to make the wood-burning furnace a downdraft furnace. Downdraft furnaces promote “secondary combustion.” Primary combustion is throttled by controlling the amount of combustion air. The unburned vapors and particulates are then mixed with heated air in a secondary combustion to burn such vapors and particulates and thereby recover more heat and reduce pollution.
“Downdraft” furnaces are known in the prior art. A downdraft furnace, also known as a “magazine” furnace, is comprised of an air supply, a wood storage section, a primary combustion chamber and ash grate, a secondary combustion chamber, a heat exchanger, an exhaust manifold, and an exhaust flue, i.e., smoke stack. Combustion is initiated at the top of the wood storage section flowing downward to the bottom of the wood storage in a primary combustion chamber. The wood combustion results in gasification of the wood fuel and combustion of the gas and residual particulates in a secondary combustion chamber resulting in a very high temperature combustion, i.e., typically 2000 degrees F. The heated air from combustion passes through the heat exchanger and exits the exhaust manifold to the smoke stack. In theory, downdraft furnaces should be able to provide complete combustion, however practical applications of the downdraft principles have resulted in incomplete combustion and poor heat transfer from fuels. The main problem arises from the difficulty of providing the proper amount of air to the combustion process of the furnace's fuel.